Recently, interests in energy storage technologies have been increasingly grown. In particular, while the application of the energy storage technologies is expanded to mobile phones, camcorders, notebook PCs, and even to electric vehicles, research and efforts for the development of the energy storage technologies have been gradually materialized.
Electrochemical devices have received most attention in the field of energy storage technologies, and there emerges an interest in rechargeable secondary batteries among these electrochemical devices.
Among the currently used secondary batteries, lithium secondary batteries, developed in the early 1990's, are spotlighted because the lithium secondary batteries may have higher operating voltage and significantly higher energy density.
An ionic conductive non-aqueous electrolyte solution, in which a salt is dissolved in a non-aqueous organic solvent, has been mainly used in a conventional secondary battery.
However, the non-aqueous electrolyte solution is disadvantageous in that the possibilities of degrading an electrode material and volatizing the organic solvent are not only high, but safety is also low due to combustion caused by an increases in ambient temperature and temperature of the battery itself. In particular, it has limitations in that gas is generated in the battery due to the decomposition of a carbonate organic solvent and/or a side reaction between the organic solvent and an electrode during charge and discharge, and thus, a thickness of the battery increases.
Since a gel polymer electrolyte proposed to improve these disadvantages of the non-aqueous electrolyte solution has excellent electrochemical stability, the thickness of the battery may not only be constantly maintained, but a contact between the electrode and the electrolyte may also be excellent due to the inherent adhesion of a gel phase, and thus, the gel polymer electrolyte is being used in the preparation of a thin-film type battery.
As a method of preparing a secondary battery in which the gel polymer electrolyte is used, the following two methods are known.
First, a polymerizable monomer and a polymerization initiator are mixed with a non-aqueous organic solvent, in which a salt is dissolved, to prepare a composition for forming a gel, the composition is injected into a battery including an electrode assembly in which a positive electrode, a negative electrode, and a separator are wound or stacked, and gelation (crosslinking) may then be performed under appropriate temperature and time conditions to prepare a secondary battery containing a gel polymer electrolyte. However, the method is disadvantageous in that wetting and safety during a heating process for the gelation are low.
As another method, there is a method in which, after surfaces of the positive electrode, negative electrode, and separator are coated with a composition for a gel polymer electrolyte, gelation is performed by using heat or ultraviolet (UV) light, a battery is then prepared by combining them, and a conventional electrolyte solution is further injected thereinto. Since the method further includes the non-aqueous organic solvent, it is not satisfactory in terms of performance as well as thermal stability of the secondary battery.
Therefore, there is a need to develop a gel polymer electrolyte having improved performance such as wetting and high-temperature stability.